Thermoelectric (TE) devices are used in a wide range of commercial, military and aerospace applications. For a nonlimiting example, applications of TE materials in NASA missions are important due to high premium of power generation in such missions to convert waste heat from turbine engines, hot sides of aircraft etc. into electric energy. Radioisotope thermoelectric generators (RTGs) were used by NASA in 25 U.S. missions (including Apollo missions to the Moon, the Viking missions to Mars, and the Pioneer, Voyager, Ulysses, Galileo, and Cassini missions to the outer solar system) since 196. However, at present commercially available TE devices typically offer limited heat to electricity conversion efficiencies, well below the fundamental thermodynamic (Carnot) limit due to limited Figure of Merit, ZT of thermoelectric materials:ZT=S2σT/κ, 
where S, σ, T, and κ=κe+κp are, respectively, the Seebeck coefficient, electrical conductivity, temperature, and thermal conductivity consisting from electron and phonon parts [H. J. Goldsmid, Thermoelectric Refrigeration, Plenum Press, New York 1964].
The ZT of the material is related to the efficiency of the TE device as:
  η  =      γ    ⁢                                        (                          1              +              ZT                        )                                1            /            2                          -        1                                          (                          1              +              ZT                        )                                1            /            2                          +                              T            hot                    /                      T            cold                              
where Thot and Tcold are temperatures (in K) of hot and cold side of the TE material.
From the 1960s to the 1990s, only incremental gains were achieved in increasing ZT, with the (Bi1-xSbx)2(Se1-yTey)3 alloy family remaining the best commercial material with ZT˜1. The breakthrough in material science in high ZT TE materials occurred in 1990s when low-dimensional materials systems [L. D. Hicks, et al., Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 53, R10493], [J. Heremans, in Thermoelectric Materials 2003—Research and Applications, MRS Symp. Proc. (Eds: G. S. Nolas, J. Yang, T. P. Hogan, D. C. Johnson), Materials Research Society Press, Pittsburgh, Pa. 2004, pp. 3-14], [T. Koga, et al., in Thermoelectric Materials—The Next Generation Materials for Small-Scale Refrigeration and Power Generation Applications, MRS Symp., MRS Press, Pittsburgh, Pa., 2000, pp. Z4.3.1-4.3.6] were shown to exhibit significantly higher ZT value than that in bulk materials. In conventional 3D crystalline systems the quantities S, σ, and κ are interrelated such as independent control of these variables to increase ZT is very difficult: an increase in S typically results in a decrease in σ, and a decrease in a produces a decrease in the electronic contribution to κ, following the Wiedemann-Franz law [A. Bejan, A. D: Allan, Heat Transfer Handbook Wiley, New York, 2003, p. 1338]. This is not the case for materials with reduced dimensionality, such as quantum wells, QW (2D), quantum wires (1D) and quantum dots, QD (0D) where introduction of a new variable (length scale) permits to decouple the aforementioned parameters and to optimize them simultaneously. For example, this is accomplished by the introduction of many interfaces, which scatter phonons more effectively than electrons, or by filtering out the low-energy electrons at the interfacial energy barriers, thus allowing the development of nanostructured materials with enhanced ZT, suitable for thermoelectric applications. However, despite of theoretical predictions and experimental demonstrations such materials are yet to found practical applications. The deficiencies of the approaches know to those skilled in the art can be understood from the following considerations:
U.S. Pat. Nos. 5,436,467 and 5,550,387 titled “Superlattice quantum well thermoelectric material” and “Superlattice quantum well material” issued to Elsner, et al. on Jul. 25, 1995 and on Aug. 27, 1996 respectively teach a multi-layer superlattice quantum well thermoelectric material using materials for the layers having the same crystalline structure (illustrated in FIG. 1). A preferred embodiment is a superlattice of Si and SiGe, both of which have a cubic structure. Another preferred embodiment is a superlattice of Boron-Carbon (B—C) alloys, the layers of which would be different stoichiometric forms of B—C but in all cases the crystalline structure would be alpha rhombohedral. Molecular Beam Epitaxy (MBE) on essentially planar substrates is disclosed as a technique for fabricating of such thermoelectric materials and devices. While thin (up to few micrometers) films of such materials can be indeed fabricated to have significantly enhanced ZT values, fabrication of much thicker layers needed for practical applications with such a technique is neither cost effective nor even feasible (material thicknesses anywhere from 10s of um to mm are required, depending on particular parameters of the TE device and its use, such as heat sink parameters, boundary resistances, etc).
U.S. Pat. No. 5,866,292 titled “Thermoelectric material” issued to Nishimoto on Mar. 23, 1999 is effectively expanding the range of semiconductor materials comprising quantum well superlattice (to FeS2 semiconductor, PbTe semiconductor and BiTe semiconductor) while offering (magnetron) sputtering on essentially planar as a preferred deposition method and providing more narrow range of quantum well thicknesses to achieve enhanced ZT. While sputtering is known as more economic process than MBE, deposition of practical thicknesses multilayer structures of acceptable quality (low defect density, etc.) is barely feasible and clearly impractical.
U.S. Pat. No. 5,886,390 titled “Thermoelectric material with diffusion-preventive layer” issued to Nishimoto on Mar. 23, 1999 teaches a quantum well multilayer with a diffusion-preventive layer being interposed between neighboring conductive layers and barrier layers. Diffusion between the conductive layers and the barrier layers under high-temperature conditions is prevented, and the thermoelectric material maintains high performance standards at high temperatures. This patent also suggests sputtering on essentially planar substrates as a deposition technique, hence, all the deficiencies of previously reviewed U.S. Pat. No. 5,866,292 are still valid.
U.S. Pat. No. 6,060,656 titled “Si/SiGe superlattice structures for use in thermoelectric devices” issued to Dresselhaus, et al. on May 9, 2000 teaches A superlattice structure for use in thermoelectric power generation systems includes m layers of a first one of Silicon and Antimony doped Silicon-Germanium alternating with n layers of Silicon-Germanium which provides a superlattice structure having a thermoelectric figure of merit which increases with increasing temperature above the maximum thermoelectric figure of merit achievable for bulk SiGe alloys. It suggests MBE deposition on essentially planar surfaces, so all arguments provided previously in relation to deficiencies of U.S. Pat. Nos. 5,436,467 and 5,550,387 are still valid.
U.S. Pat. Nos. 6,096,964 and 6,096,965 titled “Quantum well thermoelectric material on thin flexible substrate” and “Quantum well thermoelectric material on organic substrate” respectively, both issued to Ghamaty, et al. on Aug. 1, 2000 teach the thermoelectric elements having a very large number of alternating layers of semiconductor material (such as Si/SiGe) deposited on a very thin flexible substrate. These patents teach use of magnetron sputtering of quantum well superlattice on essentially planar structures, so all the provided previously arguments related to deficiencies of U.S. Pat. No. 5,866,292 are still valid.
U.S. Pat. No. 6,452,206 titled “Superlattice structures for use in thermoelectric devices” issued to Harman, et al. on Sep. 19, 2002 teaches a superlattice structure includes m monolayers of a first barrier material alternating with n monolayers of a second quantum well material with a pair of monolayers defining a superlattice period and each of the materials having a relatively smooth interface therebetween. The patent teaches the use of a material comprising a plurality of epitaxially grown (by MBE) alternating layers of materials A and B, where materials A and B are substantially lattice matched in a direction perpendicular to the direction of growth and are formed from materials which provide a thermoelectric figure of merit greater than 1.7 and which increases with increasing temperature. Since the patent suggests MBE deposition on essentially planar surfaces, so all arguments provided previously in relation to deficiencies of U.S. Pat. Nos. 5,436,467 and 5,550,387 are still valid.
U.S. Pat. Nos. 6,444,896 and 6,605,772 titled “Quantum dot thermoelectric materials and devices” and “Nanostructured thermoelectric materials and devices” respectively issued to Harman, et al. on Sep. 3, 2002 and Aug. 12, 2003 respectively teach the thermoelectric materials and devices utilizing quantum-dot superlattice (QDSL) structures to enhance ZT of the material (shown in FIG. 3). In both cases QDSL is provided on essentially planar substrate and epitaxial growth (such as with MBE) is suggested is a means for deposition of the material. Hence, all the arguments provided previously in relation to deficiencies of U.S. Pat. Nos. 5,436,467 and 5,550,387 are still valid.
U.S. Pat. No. 6,969,679 titled “Fabrication of nanoscale thermoelectric devices” issued to Okamura, et al. on Nov. 29, 2005 teaches thermoelectric material and device utilizing nanowires forming by plating into porous aluminum template. While with this technique the thickness of the thermoelectric material is limited only by the thickness of the porous alumina membrane (which can be in 10s or 100s of um as known to those skilled in the art), the main deficiency of such an approach is the insufficient density of nanowires to provide practical TE materials and devices with high cooling/heating or conversion efficiency. It is well known that porous alumina membranes of sufficient thickness (with 10s or 100s um) has pores with diameters typically exceeding 50 nm. On the other hand, it is well known for those skilled in the art that TE nanowires with <50 nm in diameter are preferable for high efficiency TE materials. While the diameter of the pores in porous alumina can be reduced by, for example, conformal coating of the pore walls, this would significantly reduce the nanowire filling fraction, indicating that the heat propagation through porous alumina host would dominate. Providing the electrical circuitry as taught by this patent to contact individual nanowires is impractical from fabrication standpoint.
U.S. Pat. No. 7,342,169 titled “Phonon-blocking, electron-transmitting low-dimensional structures” issued to Venkatasubramanian, et al. on U.S. Pat. No. 7,342,169 is teaching a thermoelectric structure comprising: a superlattice film of at least first and second material systems having different lattice constants and interposed in contact with each other; a physical interface at which said at least first and second material systems are joined with a lattice mismatch and at which structural integrity of said first and second material systems is maintained; said superlattice film of at least first and second material systems having a charge carrier transport direction normal to said physical interface and the superlattice film having a thickness of at least approximately 1.35 um wherein said superlattice film of at least first and second material systems and said physical interface comprise a superlattice structure with the lattice mismatch at said interface occurring in a plane of epitaxial growth of said at least two material systems and providing an acoustic mismatch to reduce thermal conduction across said physical interface; and orthogonally-quantum-confined superlattice phonon-blocking electron-transmitting structures. The deficiency of this patent is similar to those reviewed previously in relation to U.S. Pat. Nos. 5,436,467, 5,550,387 and 5,866,292.
Fabrication of TE materials with enhanced efficiencies of cooling/heating or conversion by means of chemical synthesis of bulk materials with nanoinclusions as described, for a nonlimiting example, in [K. F. Hsu, et al., Science 2004, 303, 818], is also included here as a reference. However, the ZT of such materials is still significantly lower than that of MBE-grown materials.
To conclude, new designs of TE materials and fabrication techniques need to be developed to realize the promise of quantum-size structure-enhancement of ZT.